As the semiconductor devices are more highly integrated recently, the processing dimension of a wiring required for forming circuits becomes finer, and a wiring has an increased number of layers. It is also required that consumption of electricity be decreased and the operating speed be increased as the semiconductor devices are more highly integrated. The increase in the resistance and the capacity of the wiring due to the increasing fineness of the wiring and the decrease in the pitch of the wiring cause a decrease in the operating speed of semiconductor devices and an increase in the consumption of electricity. Therefore, the multilayer wiring using copper (Cu) having a small electric resistance as the wiring material and a low permittivity film as the interlayer insulating film is essential for satisfying the requirements for the increased integration, the decreased consumption of electricity and the increased operating speed.
As the insulating material for the insulating film disposed between wirings and the insulating film disposed between wiring layers, it is studied that, in place of silicon oxide films formed in accordance with the chemical vapor deposition (CVD) process or the spin-on coating process which are widely used currently, low permittivity materials having smaller permittivities than those of the silicon oxide films described above such as silicon oxide films containing fluorine, silicon oxide films containing carbon and films of hydrogen silsesquioxane (HSQ), methylsilsesquioxane (MSQ), polyallyl ether (PAE) and nanoclustering silica are used. The insulating film formed with a material having a small permittivity such as those described above will be occasionally referred to as the low permittivity insulating film (the Low-k film), hereinafter.
As for the wiring material, the use of a Cu wiring comprising Cu having a small electric resistance as the main component in place of the Al wiring comprising aluminum as the main component, which is widely used currently, has been studied. Since etching of the Cu wiring is more difficult than the Al wiring, etching is conducted in accordance with the technology called the damascene process. The damascene process is roughly divided into the single damascene process and the dual damascene process.
The single damascene process is a process applied mainly to the formation of a wiring having a singly layer. A wiring groove having a prescribed wiring pattern is formed on an insulating film and, then, an electroconductive layer is formed on the insulating film in a manner such that the wiring groove is filled as an integral portion of the electroconductive layer. Then, the electroconductive layer is removed by polishing in accordance with a conventional polishing process such as the chemical mechanical polishing (CMP) to expose the insulating film, and the surface of the insulating film is made flat to form an embedded wiring.
For example, as shown in FIG. 6(a), an underlayer insulating film 12 is formed on a semiconductor substrate 11 on which elements such as transistors have been formed and, then, an etching stopper layer 13, a low permittivity insulating film 14 and a cap insulating film 15 are successively formed. Before the etching stopper layer 13 is formed, a contact plug (not shown in the figure) is formed on the underlayer insulating film 12 in a manner such that the contact plug reaches the substrate 11 although this structure is not shown in the figure.
A wiring groove 16 is formed by etching the cap insulating film 15 and the low permittivity insulating film 14 in accordance with the photolithography treatment and the etching process. Then, a barrier film 17 and a seed layer for plating comprising Cu (not shown in the figure) are successively formed by deposition on the cap insulating film 15 in a manner such that the barrier layer 17 and the seed layer coat the inner wall of the wiring groove 16. An electroconductive layer comprising Cu (not shown in the figure) is formed by deposition on the seed layer comprising Cu in accordance with the plating process in a manner such that the wiring groove 16 is filled as an integral portion of the electroconductive layer. The electroconductive layer (including the seed layer) and the barrier layer 17 are removed by polishing, and an embedded wiring of Cu (a lower wiring) 18 is formed in the wiring groove 16.
The dual damascene process is applied when a multilayer wiring structure comprising a lower wiring and an upper wiring is formed. A contact hole connected to the lower wiring and a wiring groove connected to the contact hole are formed in an insulating film in accordance with the dry etching and, then, the contact hole and the wiring groove are filled as integral portions of an electroconductive layer. The electroconductive layer is removed by polishing, and a contact plug formed by filling the contact hole and connected to the lower wiring and the upper wiring formed by filling the wiring groove are simultaneously formed.
For example, as shown in FIG. 6(b), an etching stopper layer 19, a low permittivity insulating film 20, an etching stopper layer 21, a low permittivity insulating film 22 and a cap insulating film 23 are successively formed on a lower wiring 18 formed in accordance with the single damascene process. Then, a contact hole 24 is opened by etching the cap insulating film 23, the low permittivity insulating film 22, the etching stopper layer 21 and the low permittivity insulating film 20, and a wiring groove 25 is opened by etching the cap insulating film 23 and the low permittivity insulating film 22. Thereafter, the lower wiring 18 is exposed by removing the etching stopper layer 19 by etching.
Then, as shown in FIG. 6(c), a barrier film 26 and a seed layer for plating comprising Cu (not shown in the figure) are successively formed by deposition on the cap insulating film 23 in a manner such that the formed films coat the inner walls of the wiring groove 25 and the contact hole 24. An electroconductive layer comprising Cu (not shown in the figure) is formed by deposition on the seed layer in a manner such that the wiring groove 25 and the contact hole 24 are filled as integral portions of the electroconductive layer. Then, the electroconductive layer (including the seed layer) and the barrier layer 26 are removed in accordance with the CMP process or the like so that the surface of the cap insulating film 23 is exposed In this manner, a contact plug 28 comprising Cu is formed in the contact hole 24, and an embedded wiring of Cu (an upper wiring) 29 is formed in the wiring groove 25.
In the damascene processes described above, Cu constituting the lower wiring 18 which is exposed after etching of the etching stopper layer 19 described with reference to FIG. 6(b) is oxidized and scattered by sputtering. The scattered substances comprising oxides of Cu (Cu compounds) are left remaining as etching residues on the surface of the (low permittivity) insulating films constituting the side wall of the contact hole 24 or the wiring groove 25 and on the surface of the lower wiring 18. When the upper wiring 29 and the contact plug 28 are formed as described with reference to FIG. 6(c) without removing the residues comprising the Cu compounds, the resistance of the upper wiring 29 and the lower wiring 18 is increased, and the leak current between wirings in the same wiring layer is increased due to diffusion of Cu from the Cu compounds to the low permittivity insulating films 20 and 22.
After the barrier film 26 is formed on the inner walls of the wiring groove 25 and the contact hole 24 and filled as an integral portion of the electroconductive layer comprising Cu, polishing is conducted in accordance with the CMP process to remove the excessive electro-conductive layer. After the polishing, polishing residues and polishing powder (slurry) are left remaining on the surface of the cap insulating film 23 and the upper wiring 29. The polishing powder in the fine powder condition can be removed with a jet stream of pure water or by cleaning with a brush. However, the polishing residues comprising Cu compounds such as cupper oxide (CuO) and copper hydroxide (CuOH) cannot be removed easily. When the polishing residues are left remaining, the resistance of the upper wiring 29 and the leak current between wirings in the same wiring layer is increased similarly to case where the etching residues are left remaining. In particular, when the cap insulating film 23 is a low permittivity insulating film, the leak current is increased remarkably since Cu is easily diffused from the Cu compounds.
To remove the etching residues and the polishing residues, a cleaning treatment for removing the etching residues and the polishing residues described above is conducted using a cleaning fluid comprising an alkaline or acidic aqueous solution (occasionally referred to as an aqueous solution-based cleaning agent, hereinafter).
However, as shown in FIG. 7(a), when the wiring groove 25 and the contact hole 24 in such a condition that the lower wiring 18 is exposed is cleaned using an aqueous solution-based cleaning agent, the low permittivity insulating films 20 and 22 exposed at the side wall of the wiring groove 25 or the contact hole 24 are easily invaded and etched with the aqueous solution-based cleaning agent. Due to the invasion and the etching, the low permittivity insulating films 20 and 22 exposed at the side wall of the wiring groove 25 or the contact hole 24 recede as shown by the arrows A, resulting in the condition such that the wiring groove 25 or the contact hole 24 have side walls of the eaves shape.
As shown in FIG. 7(b), when the barrier film 26 is formed so that the barrier film 26 coats the inner walls of the wiring groove 25 and the contact hole 24 in the above condition, the barrier film 26, which has the object of preventing diffusion of Cu, does not provide the sufficient coverage. Coverage of the seed layer formed on the barrier film 26 is also insufficient. Under the above condition, poor filling takes place and voids V are formed when the wiring groove 25 and the contact hole 24 are filled as integral portions of the electroconductive layer comprising Cu. Since the coverage with the barrier film 26 is insufficient, Cu diffuses into the low permittivity insulating films 20 and 22. The processing dimension of the wiring pattern changes, and short circuit takes place between adjacent wirings in the same layer and in upper and lower layers. The hygroscopic property of the low permittivity insulation film is enhanced, and the permittivity is increased. Due to the above reasons, detailed examinations are necessary for selecting the aqueous solution-based cleaning agent and deciding the conditions of the use of the agent.
Since a further decrease in the permittivity is required recently, porous materials are used as the low permittivity insulating film. When the porous low permittivity insulating film is treated with the aqueous solution-based cleaning agent, problems arise in that water in the aqueous solution-based cleaning agent is absorbed with the film and the permittivity of the film is increased, and that the aqueous solution-based cleaning agent does not penetrate into fine pores due to the surface tension, and contaminating substances in the fine pores cannot be removed.
Cleaning using a supercritical fluid of carbon dioxide which has a small surface tension and is a gas at the ordinary temperature and pressure is widely studied. For example, in Japanese Patent Application Laid-Open Nos. Heisei 10(1998)-99806 and Heisei 10(1998)-135170, methods in which inorganic contaminating substances are converted into substances soluble into a supercritical fluid of carbon dioxide by treating the substances with an acid, a base, a chelating agent, a ligand agent or an agent containing a halogen and, then, are removed by dissolving into the supercritical fluid of carbon dioxide, are described.
However, this method of cleaning has drawbacks in that conventional acids, bases and chelating agents which are used in aqueous solutions are not easily dissolved into the supercritical fluid of carbon dioxide since the supercritical fluid of carbon dioxide is apolar (hydrophobic), and that many conventional acids, bases and chelating agents exhibit the function in aqueous solutions but not in the supercritical fluid of carbon dioxide.
In particular, when this method of cleaning is applied to the cleaning of a wiring structure having Cu wiring and the low permittivity insulating film, a problem arises in that additives such as acids, bases and chelating agents and the supercritical fluid of carbon dioxide are separated from each other to cause precipitation of the additives alone on the surface of the Cu wiring and the low permittivity insulating film, and the additives in great concentrations are brought into contact with the Cu wiring and the low permittivity insulating film to cause invasion of the Cu wiring and the low permittivity insulating film. As another problem, the separated additives are not sufficiently removed (rinsed) from the surface of the Cu wiring and the low permittivity insulating film, and the contamination remains. Although the β-diketone described as an example in the above references can remove Cu oxide (CuO), metallic Cu which is not oxidized is also etched. This causes invasion of Cu in the lower wiring during the cleaning, and voids are formed.
In Japanese Patent Application Laid-Open No. 2004-249189, it is described that tertiary amines are most useful among organic amine compounds added to a supercritical fluid of carbon dioxide as the auxiliary agent for dissolution based on the reactivity of the organic amine compound and the supercritical fluid of carbon dioxide.
However, 2-diethylethanolamine, 1-dimethylamino-2-propanol and triethylamine which are described as the examples of the tertiary amine in Japanese Patent Application Laid-Open No. 2004-249189 does not dissolve copper compounds in the supercritical fluid of carbon dioxide although these amines dissolve copper compounds in aqueous solutions.